In Don’t Starve, the Night Monster appears when it’s pitch black, taking out a large chunk of your Health and Sanity. In the Caves and Ruins, this can be pretty challenging since it’s always dark. This normally puts a huge damper on exploration and such, but luckily, the game has its own ways of keeping a light when you’re underground.

Steps

1

Find some bioluminescence. Light Flowers naturally spawn in both underground realms, while Mushtrees spawn in Caves. Both are somewhat decent natural sources of light and can help light the way, but you can’t re-plant any of these resources to more convenient places. When they’re around, they’re dead useful and act as a great natural source of stationary light.

While you can’t relocate the Light Flowers, you can pick the Light Bulbs from them to craft a Lantern or refuel your Miner Hat or Lantern. You can also drop the Bulbs on the ground for some light, but its glow is so faint that they can only be used for temporary path markers or to simply keep yourself from getting killed by the Night Monster by standing directly on top of it.

2

Find some structures. There are quite a few structures in the Ruins that constantly emit light, especially in the Nightmare Cycles. The Nightmare Lights, statues, and pillars found scattered in the Ruins naturally start glowing in the Nightmare Phase, and the Ancient Pseudoscience Station lights up whenever you go near it. While this method is more appropriate for the Ruins, you can still find some structures that glow on the Caves, mainly the Nightmare Fissures, which are functionally similar to the Nightmare Lights.

You’re better off not approaching either, since the Nightmare Lights and Fissures eat away at your sanity when you stay within its glow.

You can also sometimes encounter huge shafts of light in the Caves, apparently from cracks in the ceiling reaching towards the surface. While this is technically not a structure, it’s a permanent source of light you can stay in whenever you need to.

3

Craft yourself some light. The most common and reliable way to get some light in this dark and dreary areas is to build your own lights. The easiest to build are basic Torches, Campfires, and Fire Pits, but there are other ways to get some light shining. Do note that you need to constantly refuel your light sources (save for the Pumpkin Lantern and the Torch) or else they’ll go out.

Miner Hats are by far the most reliable way to get some light shining, and costs a Straw Hat, 1 Gold Nugget and 1 group of Fireflies. You can learn the recipe on the Alchemy Engine. The drawback of having one is that you won’t be able to equip a helmet. The item lasts for almost 8 minutes, but you can refuel it with Fireflies, Slurtle Slime, and Light Bulbs to keep it going indefinitely.

Lanterns are also pretty good to bring, though it occupies your hand slot. You can drop your Lantern on the ground and it will keep burning as long as you have fuel. You can learn the recipe from the Alchemy Engine, and costs 2 Light Bulbs, 3 Twigs, and 2 Ropes. Its light lasts for 8 minutes, can be turned on and off to conserve fuel, and can also be refueled using Fireflies, Slurtle Slime, and Light Bulbs. Be careful though, Splumonkeys can steal this when you place it, robbing you of precious light.

You can also craft a Pumpkin Lanterns, which costs 1 Pumpkin and 1 Firefly, and you can learn the recipe on the Alchemy Engine. This light source lasts a bit longer, but spoils over time and cannot be refueled. It lasts about 10 minutes in the Summer and 12 ½ minutes in Winter.

In RoG, it lasts 7 ½ minutes in the Summer, 10 minutes in Spring and Autumn, and 12 ½ minutes in Winter.

Lastly, in RoG you can craft Moggles with 2 Moleworms, 2 Electrical Doodads, and 1 Glow Berry, and you can learn the recipe on the Alchemy Engine. It’s somewhat unusual in that it gives you full night-vision with a sort of negative filter over the screen, and you can even see the entire screen instead of a radius of light around you. The Moggles last for 8 minutes before breaking, and you can refuel it with Glow Berries whenever you need to.